1. Field of the Invention.
This invention relates to a process of pyrolytically forming a metal oxide coating on an upper face of a hot glass substrate in sheet or ribbon form during its conveyance in a downstream direction along a path leading beneath a downwardly opening coating chamber, in which process said coating is formed from coating precursor vapour and an oxidising gas which are fed in the downstream direction along a passageway of said coating chamber to which said substrate face is exposed. The invention also relates to apparatus for use in pyrolytically forming a metal oxide coating on an upper face of a heated glass substrate in sheet or ribbon form, said apparatus comprising conveyor means for conveying a said substrate in a downstream direction along a path and a roof structure defining a coating chamber opening downwardly onto said path and comprising a passageway along which coating precursor vapour and oxidising gas can be conducted downstream, in contact with a said upper substrate face during said conveyance of the substrate.
2. Description of the Related Art.
Such processes and apparatus are useful in the manufacture of coated glass for various purposes, the coating being selected to confer some particular desired property on the glass. Especially important examples of coatings which may be applied to glass are those designed to reduce the emissivity of the coated face in respect of infra-red radiation, especially infra-red radiation having wavelengths in excess of 3.mu.m, and those designed to reduce the total energy transmissivity of the coated glass in respect of solar radiation. It is known, for example, to provide glass with a low infra-red emissivity coating of tin dioxide for heat conservation purposes, and it is also known to provide glass with a solar energy transmissivity reducing coating of a metal oxide such as titanium dioxide or of a mixture of metal oxides such as Fe.sub.2 O.sub.3 =CoO=Cr.sub.2 O.sub.3 with the principal object of reducing solar heat gain or glare.
Because the coatings are usually applied to a thickness of between about 30 nm and 1200 nm, depending on the nature of the coating material and the properties required, variations in the thickness of a coating will not only mean that the required infra-red emissivity or energy transmissivity is not uniformly acheived, but also that objectionable interference effects may occur. A regular and uniform thickness is therefore important for good optical quality as well as for achieving the required emissivity or transmissivity. It will be apparent that coatings which are applied to glass to be used for glazing purposes should have a high and uniform optical quality. The coatings should therefore also be free from stains and other localised defects.
It is known to deposit coatings from coating precursor material in the vapour phase as opposed to the liquid phase, and that this can promote freedom from localised defects. This freedom from localised defects is achieved by directing separate streams of highly concentrated vaporised coating precursor and oxidising gas towards the substrate so that they mix and react only while in contact with the substrate, so that the oxide is formed directly onto that substrate and not in the atmosphere above it whence particles of coating material could fall onto the substrate to become incorporated into the coating as defects. The vapour laden atmosphere is then aspirated away from the substrate before cooled precursor vapour or reaction products formed in the atmosphere out of contact with the substrate can deposit as defects on or in the coating being formed.
Known vapour phase coating techniques have not resulted in the formation of coatings which have a regularity of thickness which is sufficient to meet ever more exacting commercial quality requirements especially for large glazing sizes as are increasingly demanded by modern architectural practice. Attempts have been made to introduce a concentrated stream of coating precursor vapour into the coating chamber uniformly in time and over the entire width of the substrate to be coated and more volatile coating precursor materials have been selected in efforts to facilitate this. Various steps have also been taken to modify known techniques in order to ensure that the coating precursor vapour flows in a carefully controlled, turbulence free manner in contact with the substrate during coating formation. Unfortunately, it has been found impossible to exercise the degree of control required over the introduction of the vapour and its behavior in the coating chamber when operating on a commercial scale, with the result that unpredictable thickness variations occur in the coating and a proportion of the coated glass produced is not of an acceptable quality.